1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus for transforming incident light to a corresponding electric signal for picking up the image of a desired field, and more particularly to a solid-state image pickup apparatus for producing multiple fields of image signals without resorting to an overflow drain (OFD) modulation or similar control. The present invention is desirably applicable to, e.g., an electronic still camera, an image input apparatus, a movie camera or a cellular phone by way of example.
2. Description of the Background Art
Conventional solid-state image pickup apparatuses include an electronic still camera of the type having a mechanical shutter and operable in an interlace read mode, which is capable of transferring a greater amount of signal charges than a progressive read mode. In order to make up for a decrease in the number of electrons ascribable to the higher density of pixels of a photosensitive cell array to cause its pixel size to decrease, a current trend with this type of image pickup apparatus is toward the configuration for producing multiple fields of image signals, so-called the multiple field configuration. The multiple field configuration involves however a problem that signal charges reserved for forming a second and a third field are stored in the photodiodes of the photosensitive array for a long period of time after the closure of the mechanical shutter. As a result, such signal charges are apt to decrease or deteriorate due to thermal saturation diffusion.
In light of the above, it is a common practice to execute OFD modulation with a method taught in, e.g., U.S. patent application publication No. US 2001/0010553 A1 for thereby increasing the amount of signal charge to remain at the end of exposure. More specifically, the above method controllably drives a CCD (Charge Coupled Device) image sensor by lowering an OFD voltage during exposure in order to increase the amount of charge to be stored, thereby canceling a decrease in the amount of stored signal charge ascribable to thermal saturation diffusion that would occur after the mechanical shutter has been closed on the elapse of exposure time. This method is generally considered to effectively accomplish the multiple field configuration.
Japanese patent laid-open publication No. 2003-32549 proposes to execute OFD modulation in a progressive read mode with a CCD image sensor, which is usable for both of an interlace read mode and a progressive read mode.
The problem with the two prior art schemes described above is that control over the OFD voltage is essential and makes control over image sensing devices sophisticated.